November 3, 2009

Potomac Creek Bridge Mock-Up


I was in the mood for some CAD time tonight, so I started the drawings for Potomac Creek Bridge. This mock-up is to help me decide which type of bridge to build. I currently think the truss will be the best bet, but the bean pole and corn stalk trestle has a strong historical appeal- any bridge that amazed Abe Lincoln has got to be cool.


What do you think, truss or bean pole trestle? To refresh your memory, the bean pole is similar to the picture in the blog header image.



This mock up is essentially a half scale version of the actual bridge. The dimensions of the truss are 2.5 inches tall and 48 inches long in three arched sections. The parts will be all laser cut with precise drilled holes for the pins, so it will assemble fairly easily. The arch will be cut from 1/8th inch plywood and then laminated together to get the proper thickness and spacing for the vertical members. Not all parts are shown in the mock-up.

The piers will also be laser cut plywood and then faced with plasticard stone.

I had a Eureka moment last night. As I studied the bridge photos and started drawing the bridge parts it occurred to me how they built the truss bridge. I now believe that they built the truss over the existing trestle. The truss uses mostly identical parts that could be lowered in place with a light crane, possibly mounted on a flat car so it could be moved. These would be nailed or bolted together piece-by-piece while the existing bridge hosted traffic. The lower story of the trestle would have supported the chords as they built them out. Essentially the trestle would act as scaffolding for the truss as they erected it. Once the two walls of the truss were completed and able to bear weight, they would have removed the trestle sticks, if any, that were in the way to allow the diagonal sway braces to be installed.

Now, how about this as a scene, showing the bridge in a state of construction? The pencil sketch posted earlier provides a clue on how it would look.

2 comments:

  1. Do you already have a truss elsewhere? What about a bean pole?

    Personally, I want to see the bean pole, just because it'd be a masterpiece when done.

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  2. No, I have a smaller trestle at Clairborne Run. See the earlier postings for pics of that.I will also have a significant amount of low trestles at the approach to Aquia harbor, as well as the large pier.

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